Four-channel stereophonic reproducing system for reproducing discrete CD-4 four-channel stereo disc

ABSTRACT

There is provided a four-channel stereophonic reproducing system wherein each muting circuit comprises a limiter for limiting the carrier level of a frequency-modulated signal to a predetermined level, a control circuit actuated by the output of the limiter and a switching circuit controlled by the control circuit, whereby when the carrier level of the frequency-modulated signal decreases, the detected output of the sub-channel signal including a large amount of noise components is blocked by the muting circuit.

The present invention relates to a four-channel stereophonic reproducingsystem for reproducing discrete CD-4 four-channel stereo discs.

As in a conventional CD-4 system four-channel record disc, the leftstereophonic composite signal is recorded on the left wall of aso-called 45--45 system groove. The left stereophonic composite signalcomprises the sum of a first main channel signal and a first sub-channelsignal, the first main channel signal consisting of the sum of first andsecond channel signals (CH1 + CH2) and the first sub-channel consistingof the frequency-modulated difference between the first and secondchannel signals F (CH1 - CH2). Similarly, the right stereophoniccomposite signal is recorded on the right wall of the 45--45 systemgroove. The right stereophonic composite signal comprises the sum of asecond main channel signal and a second sub-channel signal, the secondmain channel signal consisting of the sum of third and fourth channelsignals (CH3 + CH4) and the second sub-channel signal consisting of thefrequency-modulated difference between the third and fourth channelsignals F (CH3 - CH4).

Accordingly, when reproducing the record disc, the left and rightstereophonic composite signals are picked up by a cartridge and the mainchannel signals separated from the associated sub-channel signals bymeans of filters to detect the sub-channel signals. The detectedsub-channel signals are then added to and subtracted from the mainchannel signals to obtain four separate output signals corresponding tothe first to fourth channel signals.

It is an object of the present invention to provide an improvedfour-channel stereophonic reproducing system comprising a muting circuitfor detecting the carrier level of the frequency-modulated signals,whereby when the detected carrier level is lower than a predeterminedvalue, the detected FM output is prevented from appearing at the outputterminal to thereby prevent deterioration of the signal-to-noise ratiodue to the decreased carrier level.

The above and other objects, features and advantages of the presentinvention will become readily apparent from considering the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram showing an embodiment of a four-channelstereophonic reproducing system according to the present invention;

FIGS. 2a and 2b are output voltage waveform diagrams of the limiter usedin the embodiment of FIG. 1;

FIGS. 2c and 2d are output voltage waveform diagrams of thedifferentiation circuit used in the embodiment of FIG. 1;

FIG. 3 is a schematic circuit diagram for the embodiment shown in FIG.1;

FIG. 4 is a schematic circuit diagram showing another embodiment of thepresent invention; and

FIG. 5 is a schematic circuit diagram showing still another embodimentof the present invention.

The present invention will now be described in greater detail withreference to the illustrated embodiments.

Referring first to FIG. 1 showing a first embodiment of the invention,numeral 1 designates a cartridge unit, 2 a reproducing section for theleft channel signal picked up by the cartridge unit 1, 3 a reproducingsection for the right channel signal picked up by the cartridge unit 1.In the embodiment of FIG. 1, the right channel signal reproducingsection 3 is identical in construction and operation with the leftchannel signal reproducing section 2. Therefore, no detailed blocks ofthe section 3 are shown and its operation will not be described in thediscussion to follow. Numeral 4 designates a preamplifier, 5 a mainchannel circuit including a low-pass filter for passing only the mainchannel signal component in the stereophonic composite signal, 6 amatrix circuit, 7 and 8 the output terminals of the matrix circuit 6.The main channel circuit 5 and the matrix circuit 6 constitute a mainchannel signal reproducing section.

Numeral 9 designates a high-pass filter for passing only the sub-channelsignal component in the stereophonic composite signal, 10 a limiter forlimiting the amplitude of the sub-channel signal obtained through thehigh-pass filter 9, 11 an FM detector circuit, 12 a switching circuit,13 a differentiation circuit connected to the output terminal of thelimiter 10, 14 a control circuit for controlling the switching circuit12 with the output of the differentiation circuit 13. The high-passfilter 9, the limiter 10, the FM detector circuit 11, the switchingcircuit 12, the differentiation circuit 13 and the control circuit 14constitute a sub-channel signal reproducing section, and a mutingcircuit is comprised of the limiter 10, the differentiation circuit 13,the control circuit 14 and the switching circuit 12. Numerals 15 and 16designate the output terminals of the matrix circuit in the rightchannel signal reproducing section 3.

The embodiment of FIG. 1 operates as follows. The left and rightstereophonic composite signals picked up by the cartridge unit 1 areapplied respectively to the left and right channel reproducing sections2 and 3. The left side stereophonic composite signal applied to the leftchannel reproducing section 2 is amplified by the preamplifier 4, andthe main channel signal is then passed through the low-pass filter inthe main channel circuit 5 and applied to the following matrix circuit6. On the other hand, the sub-channel signal is passed through thehigh-pass filter 9 and it is then applied to the limiter 10. The outputof the limiter 10 is detected by the detector circuit 11 and applied tothe switching circuit 12, and it is also applied to the differentiationcircuit 13.

Assuming now that the carrier level of the frequency-modulated signal issufficiently high, the limiter 10 produces an output as shown in FIG. 2aand the differentiation circuit 13 produces an output as shown in FIG.2c. In this case, therefore, the control circuit 14 produces asufficiently high output and the switching circuit 12 is turned on.Consequently, the sub-channel signal is applied to the matrix circuit 6where it is matrixed against the main channel signal, and thus the frontand rear signals are produced at the output terminals 7 and 8.

On the other hand, when the carrier level of the FM signal becomes lowerthan a predetermined value, the limiter 10 cannot perform its amplitudelimiting function satisfactorily and thus it produces an output as shownin FIG. 2b. In this case, therefore, the differentiation circuit 13produces an output as shown in FIG. 2 and the output of the controlcircuit 14 becomes extremely low. Consequently, the switching circuit 12is turned off and the output of the detector circuit 11 is not suppliedto the matrix circuit 6.

FIG. 3 illustrates an exemplary circuit construction of the embodimentshown in FIG. 1, and in FIG. 3 the same reference numerals as used inFIG. 1 designate the identical component parts. The output signal of thelimiter 10 is applied to an input terminal 17 so that it isdifferentiated by the differentiation circuit 13 comprising a capacitor18 and a resistor 19 and it is then applied to the control circuit 14.In the control circuit 14, the differentiator output is detected by adiode 20 and it is then used to charge an integrating capacitor 21. Thiscapacitor voltage is applied through a resistor 22 to the base of afirst transistor 23 to turn it on and off.

When the carrier level of the FM signal is sufficiently high so that thedifferentiation circuit 13 produces the output shown in FIG. 2c, thefirst transistor 23 is turned on by the voltage on the capacitor 21 sothat its collector potential decreases and thus the base potential of asecond transistor 24 decreases to turn the second transistor 24 off.Consequently, the collector potential of the second transistor 24increases and it raises through a resistor 25 the gate potential of anFET 26 constituting the switching circuit 12. When this occurs, the FET26 is turned on and the output signal of the detector circuit 11 whichwas applied to the source of the FET 26 is passed to its drain. Thedetector output signal thus passed through the FET 26 is applied to thematrix circuit 6 through a resistor 27 and a capacitor 28 and through anoutput terminal 29.

On the contrary, when the carrier level of the FM signal is lower thanthe predetermined value so that the differentiation circuit 13 producesthe output shown in FIG. 2d, the voltage on the capacitor 21 is low andthe first transistor 23 is turned off. As a result, the secondtransistor 24 is turned on and the gate potential of the FET 26decreases to turn it off. The output signal of the detector circuit 11is thus blocked and it is not applied to the matrix circuit 6.

In FIG. 3, numerals 30, 31, 32, 33, 34 and 35 designate bias resistors.

It should be appreciated that with the abovedescribed embodiment, whenthe carrier level of the FM signal is sufficiently high, any noise maybe practically eliminated by the amplitude limiting action of thelimiter 10, whereas when the carrier level decreases below thepredetermined value so that the amplitude limiting action is madepractically inoperative, the switching circuit 12 is turned off to blockthe detector output itself. In either of these cases, therefore, animproved reproduction with a higher degree of freedom from noise may beensured.

FIG. 4 illustrates a circuit diagram showing another embodiment of thepresent invention, in which the same reference numerals as used in FIGS.1 and 3 designate the identical or equivalent component parts.

The embodiment of FIG. 4 differs from the embodiment shown in FIGS. 1and 3 in that the switching circuit is comprised of a known type ofAutomatic Noise Reduction System (ANRS) designed to block the detectoroutput and reduce the amount of noise when the signal level becomes low,and in this way the detector output is blocked when the carrier level ofthe FM signal decreases below a predetermined value.

In other words, the output signal of the limiter 10 is applied to boththe differentiation circuit 13 and the detector circuit 11, and thedetected output of the detector circuit 11 is applied to both the matrixcircuit 6 and the ANRS 36.

When the signal level at the output terminal of the detector circuit 11is sufficiently high, a third transistor 38 is turned on through acapacitor 37, and a large negative potential is applied to the base of afourth transistor 41 by detector diodes 39 and 40 to turn it off.Consequently, the output of the detector circuit 11 is not applied tothe fourth transistor 41, but it is applied to the matrix circuit 6where it is matrixed against the main channel signal from the mainchannel circuit 5 to produce the front and rear signals at the outputterminals 7 and 8.

On the other hand, when the signal level is low, the third transistor 38is turned off and the +B voltage is applied to the base of the fourthtransistor 41 to turn it on. As a result, the output of the detectorcircuit 11 is grounded through a capacitor 42, a resistor 43 and thefourth transistor 41 and it is not supplied to the matrix circuit 6. InFIG. 4, numerals 44, 45, 46, 47 and 48 designate resistors, and numerals49 and 50 designate capacitors.

While the ANRS 36 operates in the manner described above, when thecarrier level is sufficiently high so that the differentiation circuit13 produces the output shown in FIG. 2c, the third transistor 38 isturned on and the fourth transistor 41 is turned off in the ANRS 36,thus supplying the output signal of the detector circuit 11 to thematrix circuit 6.

On the other hand, when the carrier level of the FM signal is lower thanthe predetermined value so that the differentiation circuit 13 producesthe output shown in FIG. 2d, the third transistor 38 is turned off andthe fourth transistor 41 is turned on, and thus the detector output isgrounded through the fourth transistor 41 and not applied to the matrixcircuit 6.

It will thus be seen that the embodiment of FIG. 4 employs the ANRS 36in place of the switching circuit 12 to produce similar results to thoseproduced by the embodiment shown in FIGS. 1 and 3.

FIG. 5 illustrates still another embodiment of the invention, and thecomponent parts identical in function with those of the embodimentsshown in FIGS. 1, 2 and 4 are designated by the same reference numerals.This embodiment differs from the first two embodiments in that themuting function is performed by not only detecting the presence of thecarrier, but also in accordance with the magnitude of the carrier wave.

In FIG. 5, numeral 51 designates a detecting circuit for detecting thepresence of a carrier wave, 52 a DC amplifier which serves the functionof amplifying the output signal of the detecting circuit 51 in additionto its switching function. Numeral 53 designates a high speed mutingcircuit whereby when there is no carrier wave, the output of thedetector circuit 11 is grounded through a capacitor 54 to block theapplication of the detector output to the matrix circuit 6. Numeral 55designates a long time muting circuit whereby when the carrier wave isintermittently and continually off so that it is impossible for the highspeed muting circuit 53 to perform the muting function accurately, thelong time muting circuit 55 is switched into operation by the DCamplifier 52 to perform the muting function. In other words, the longtime muting circuit 55 comprises a capacitor 56, a diode 57, a timeconstant circuit 60 having a long time constant and comprised of aresistor 58 and a capacitor 59, a fifth transistor 61 and a resistor 62,whereby when the carrier wave has been off for several times, thecapacitor 59 is charged sufficiently to turn the fifth transistor 61 on.Accordingly, when a situation arises continually where the output signalof the DC amplifier 52 includes no carrier wave so that it is impossiblefor the high speed muting circuit 53 to respond to the output signal ofthe DC amplifier 52 satisfactorily, the long time muting circuit 55 isbrought into operation and the fifth transistor 61 is turned on toground the output signal of the detector circuit 11 through thecapacitor 54 and the fifth transistor 61 and thereby prevent theapplication of the detector output to the matrix circuit 6.

In the present embodiment, the fifth transistor 61 of the long timemuting circuit 55 is also utilized to construct the switching circuit.

In other words, when the carrier level is sufficiently high so that thedifferentiation circuit 13 produces the output signal shown in FIG. 2c,a sixth transistor 63 constituting the control circuit 14 is turned onand the fifth transistor 61 of the long time muting circuit 55 is turnedoff, thus supplying the output signal of the detector circuit 11 to thematrix circuit 6.

On the other hand, when the carrier level is lower than thepredetermined value so that the differentiation circuit 13 produces theoutput shown in FIG. 2d, the sixth transistor 63 is turned off and the+B voltage is applied to the base of the fifth transistor 61 through theresistors 31 and 64 to turn it on. Consequently, the output of thedetector circuit 11 is grounded through the capacitor 54 and the fifthtransistor 61 and the application of the detector output to the matrixcircuit 6 is prevented.

With the arrangement described above, it is possible to obtain the sameresults as the embodiment shown in FIGS. 1 and 3.

While, in the embodiments of this invention described hereinbefore, theoutput of the limiter 10 is differentiated by the differentiationcircuit 13, this is done with the intention of increasing the range ofvariation of the output signal of the limiter 10 to prevent anyerroneous operation, and therefore it would be apparent to those skilledin the art that the same results may be obtained without the provisionof the differentiation circuit 13, if the circuitry is designedsuitably.

What we claim is:
 1. A four-channel stereophonic reproducing system forreproducing a discrete CD-4 four-channel stereo disc wherein left andright stereophonic composite signals are recorded on the left and rightwalls of the groove on said disc, said left stereophonic compositesignal comprising the sum of a first main channel signal and a firstsub-channel signal and said right stereophonic composite signalcomprising the sum of a second main channel signal and a secondsub-channel signal, said first main channel signal comprising the sum offirst and second channel signals and said second main channel signalcomprising the sum of third and fourth channel signals, said firstsub-channel signal comprising the frequency-modulated difference betweensaid first and second channel signals and said second sub-channel signalcomprising the frequency-modulated difference between said third andfourth channel signals, comprisingmeans for reproducing a main channelsignal and a sub-channel signal having a carrier from each of left andright stereophonic composite signals recorded on said disc; detectormeans for detecting said sub-channel signals reproduced by saidreproducing means; matrix means for matrixing said main channel signalsreproduced by said reproducing means and output signals from saiddetector means to produce four discrete stereophonic audio signals;switching means provided between said detector means and said matrixmeans; level discriminating means for detecting when the carrier levelof a sub-channel signal is lower than a predetermined value; and controlmeans controlled by the output of said level discriminating meanswhereby, when said carrier level becomes lower than said predeterminedvalue, said switching means is rendered nonconductive to preventapplication of the output of said detector means to said matrix means.2. A system according to claim 1, wherein said level discriminatingmeans comprises a pair of detecting units each connected to an input ofsaid detector means.
 3. A system according to claim 1, wherein saiddetecting means comprises a pair of detecting units each thereofconsisting of a limiter connected to an input of said detector means,and a differentiation circuit connected to the output of said limiter.4. A system according to claim 1, wherein said control means comprises apair of control units each thereof consisting of a diode coupled to theoutput of said level discriminating means, a capacitor charged by theoutput of said diode, a first transistor having the base thereofconnected to said capacitor, and a second transistor having the basethereof connected to the collector of said first transistor, and whereinsaid switching means comprises a pair of switching units each thereofconsisting of a field effect transistor having its gate connected to thecollector of said second transistor, its source connected to an outputterminal of said detector means and its drain connected to an inputterminal of said matrix means.
 5. A four-channel stereophonicreproducing system for reproducing a discrete CD-4 four-channel stereodisc wherein left and right stereophonic composite signals are recordedon the left and right walls of the groove in said disc, said leftstereophonic composite signal comprising the sum of a first main channelsignal and a first sub-channel signal and said right stereophoniccomposite signal comprising the sum of a second main channel signal anda second sub-channel signal, said first main channel signal comprisingthe sum of first and second channel signals and said second main channelsignal comprising the sum of third and fourth channel signals, saidfirst sub-channel signal comprising the frequency-modulated differencebetween said first and second channel signals and said secondsub-channel signal comprising the frequency-modulated difference betweensaid third and fourth channel signals, comprisingmeans for reproducing amain channel signal and a sub-channel signal having a carrier from eachof left and right stereophonic composite signals recorded on said disc;detector means for detecting said sub-channel signals reproduced by saidreproducing means; matrix means for matrixing said main channel signalsreproduced by said reproducing means and output signals from saiddetector means to produce four discrete stereophonic audio signals;switching means provided between said detector means and said matrixmeans; level discriminating means for detecting when the carrier levelof said sub-channel signal is lower than a predetermined value; andautomatic noise reduction means responsive to the output of saiddetector means to prevent the application of the output of said detectormeans to said matrix means when the signal level of the output of saiddetector means is low, whereby when the carrier level of saidsub-channel signal becomes lower than said predetermined value, saidautomatic noise reduction means is actuated by the output of saiddetecting means to prevent the application of the output of saiddetector means to said matrix means.
 6. A system according to claim 5,wherein said level discriminating means comprises a pair of limiterseach connected to an input of said detector means.
 7. A system accordingto claim 5, wherein said detecting means comprises a pair of detectingunits each thereof consisting of a limiter connected to an input of saiddetector means, and a differentiation circuit connected to the output ofsaid limiter.
 8. A system according to claim 5, wherein said automaticnoise reduction means comprises a pair of automatic noise reductionunits each thereof consisting of a third transistor having the basethereof connected to an output terminal of said detector means, and afourth transistor having its collector connected to the output terminalof said detector means and its base connected to the collector of saidthird transistor through a detector diode, and wherein the outputterminal of said level discriminating means is connected to the base ofsaid third transistor.
 9. A four-channel stereophonic reproducing systemfor reproducing a discrete CD-4 four-channel stereo disc wherein leftand right stereophonic composite signals are recorded on the left andright walls of the groove in said disc, said left stereophonic compositesignal comprising the sum of a first main channel signal and a firstsub-channel signal and said right stereophonic composite signalcomprising the sum of a second main channel signal and a secondsub-channel signal, said first main channel signal comprising the sum offirst and second channel signals and said second main channel signalcomprising the sum of third and fourth channel signals, said firstsub-channel signal comprising the frequency-modulated difference betweensaid first and second channel signals and said second sub-channel signalcomprising the frequency-modulated difference between said third andfourth channel signals, comprisingmeans for reproducing a main channelsignal and sub-channel signal having a carrier from each of left andright sterephonic composite signals recorded on said disc; detectormeans for detecting said sub-channel signals reproduced by saidreproducing means; matrix means for matrixing said main channel signalsreproduced by said reproducing means and output signals from saiddetector means to produce four discrete stereophonic audio signals;level discriminating means for detecting when the carrier level of saidsub-channel signal is lower than a predetermined value; muting meansconnected to the output of said level discriminating means for detectingthe presence of carrier whereby, when there is no carrier wave in theoutput of said level discriminating means, said muting means is actuatedto prevent the application of the output of said detector means to saidmatrix means; and control means controlled by the output of said leveldiscriminating means for actuating said muting means to prevent theapplication of the output of said detector means to said matrix meanswhen said carrier level becomes lower than said predetermined value. 10.A system according to claim 9, wherein said level discriminating meanscomprises a pair of limiters each connected to an input of said detectormeans.
 11. A system according to claim 9, wherein said leveldiscriminating means comprises a pair of limiters each connected to aninput of said detector means, and a differentiation circuit connected tothe output of said limiter.
 12. A system according to claim 9, whereinsaid muting means comprises a pair of muting units each thereofconsisting of means for detecting the presence of carrier wave in theoutput signal of said level discriminating means, a diode for detectingthe output of said carrier detecting means, and a fifth transistorhaving its collector connected to an output terminal of said detectormeans, its base connected to said diode and its emitter connected toground, and wherein said control means comprises a pair of control unitseach thereof consisting of a diode for detecting the output of saidlevel discriminating means, a capacitor charged by the output of saiddiode and a sixth transistor having its base connected to said capacitorand its collector connected to the base of said fifth transistor.